In order to ensure the operation of power plants even in the event of failure of the primary fuel, secondary fuels are stored as emergency fuel in the proximity of the power plant in order to be able to continue to operate the power plant in for example an emergency operating state when need arises. Depending on design of the power plant and demands from the electricity supply network it may be necessary for the power plant to be required to have the capability of being operated in the emergency operating state using a secondary fuel for between 7 and 14 days.
In the case of combined gas and steam power plants (CCPP), gas, especially natural gas, which is directed to the power plant via a supply line, such as pipelines, is used as primary fuel. Oil (diesel), which is stored in tanks on the site of the power plant, is as a rule used as secondary fuel. If a failure of the gas supply via the supply line occurs, a switch can be made to oil as secondary fuel. In order to be able to store oil in sufficient quantities, access to supply logistics is necessary.
In order to be able to combust oil in the gas turbine, additional burners are required in the gas turbine, which makes the development of such a gas turbine more expensive. By the same token, additional auxiliary systems, such as pumps and measurement and control systems, are required on the gas turbine. Also, large tanks are required on the site of the power plant.
Oil as secondary fuel in addition to gas as primary fuel has a number of disadvantages. In particular, oil has a lower level of efficiency compared with gas when being combusted in the gas turbine since lower flame temperatures can be achieved using oil. This leads to a significant power reduction during firing with oil. Therefore, such a power plant has to be of larger design in order to be able to achieve total rated power even during emergency operation.
The operating stability when using oil as fuel is also lower than in the case of gas. Also, if the power is lower as a result of firing with oil, the emissions increase when firing with oil. This, depending on local environmental regulations, necessitates an additional catalyst. Using oil as secondary fuel, regulatory authorizations therefore also increase.
One of the most problematic disadvantages during operation, however, is that a relatively long changeover period is required for switching from gas to oil. Consequently, oil as emergency fuel is suitable particularly only for scheduled outages. In the case of sudden failures the power plant would have to be run down, depending on gas turbine type.